1. Field of the Invention
The present invention relates to a liquid level indicator suitable for use in high pressure liquefied gas tanks, and more specifically to a liquid level indicator using a laser beam applicable to high pressure liquefied gas tanks such as liquefied petroleum gas tanks, liquefied chlorine gas tanks, liquefied ammonia tanks, liquefied nitrogen gas tanks, etc.
2. Description of the Prior Art
As a prior-art liquid level indicator for detecting a liquid level in an airtightly closed liquid vessel, there exists an indicator in which two pressure-proof glass plates with two opposing slits are fixed to the outside of a vessel and fastened together by flanges, the interior of the slits being in communication with the inside of the vessel so that the liquid level can be detected by visual inspection of the liquid in the slits. In the prior-art liquid level indicator of this type, however, there exists a problem that this indicator is not available for a combustible liquefied gas vessel such as liquefied petroleum gas tank, because the glass indicator is fragile.
On the other hand, there is known another prior-art liquid level indicator in which a float is floated on a liquid level within a vessel with a rope connected to the float being guided via a wheel to a weight hanging at the outside of the vessel in balance each other so that the float position can be detected visually from the outside of the vessel by the position of the weight. In this method, however, there exists a problem that it is difficult to well balance the two due to the rope resistance produced when the rope is guided through a sealing aperture formed in the vessel. Further, it is difficult to manufacture a level indicator of this type at low cost, when an internal pressure is applied to the vessel.
Further, there exits another prior-art method of detecting a liquid level by measuring electrostatic capacitance formed between two electrode plates the first located at the upper portion of a vessel the second floated on a liquid level. However, in case of combustible liquid, the electric devices should be constructed in a explosion-proof type, thus resulting in a higher cost.
On the other hand, recently optical technology using an optical fiber has progressed markedly and also a shutter which can pass a light pulse as short as several to several tens of nanoseconds (10.sup.-9 sec.) has been developed. Furthermore, it has recently become possible to measure time by an order of 10.sup.-9 sec. Therefore, it has become possible to accurately measure a short distance by a light wave distance-measuring means which has been so far used to measure a long distance of several kilometers or more.